This invention relates to the field of electronic instruments for measuring the polarization state of a beam of light and, more particularly, to such instruments that are capable of detecting effects on the polarization state of an incident light beam caused by an optical device under test (i.e., an optical system, subsystem, or component). Specifically, one embodiment of the invention provides a method and apparatus for impinging light beams having predetermined wavelengths and states of polarization on an optical device under test to ascertain a response characteristic of the optical device to different polarization states, thereby determining the polarization mode dispersion in the optical device.
Accurate characterization of optical devices is becoming increasingly important as optical devices become more complex and applications for optical devices proliferate, for example, in fiber optic telecommunications. One of the fundamental specifications of any optical device with an optical input and an optical output is polarization dispersion. Dispersion is a general term which denotes the tendency of an optical pulse to spread out in time as it propagates through an optical transmission medium. Several varieties of dispersion can be measured in optical fibers. For example, chromatic dispersion arises because different optical wavelengths travel at different velocities, so that a pulse comprising a finite spectrum of optical frequencies is gradually smeared out in time by propagation along an optical fiber. Similarly, polarization mode dispersion arises because different optical polarizations can travel at different velocities. Polarization mode dispersion can limit the available transmission bandwidth in fiber optic transmission links.
Conventionally, one technique for measuring polarization mode dispersion involves a device resembling a Michelson interferometer, shown in FIG. 1 and in K. Mochizuki, Y. Namihira, and H. Wakabayashi, "Polarization mode dispersion measurements in long single mode fibers," Elect. Lett., 17, 1981, pp. 153-154. Light from a source with a short coherence length is directed through an arrangement of mirrors, polarizers, and a beamsplitter which enables generation of a beam of light composed of two orthogonal polarizations which have experienced a variable relative time delay. These two polarizations are launched into the device under test so that they match the input principal states of polarization of the device. Light exiting the device is passed through a polarizer oriented midway between the output principal states of polarization and is then detected. Cross-correlation between the two orthogonal signals is thereby apparent from the level of visibility of optical fringes at the detector. Polarization mode dispersion in the device under test causes a shift in the delay corresponding to maximum visibility, and this time shift is .tau..sub.PMD.
However, this technique has several disadvantages. The principal states of polarization must be known or found for this technique to work, but the apparatus does not lend itself to a search for the principal states. Moreover, the requirement of a short coherence length implies a broad spectrum. Many test devices of interest have principal states and .tau..sub.PMD which are strong functions of wavelength, and such devices simply cannot be measured using this technique because the required short coherence length of the optical source implies a wide spectrum.
A second polarization mode dispersion measurement technique requires a tunable optical source and a polarimeter. The setups described in two references, N. S. Bergano, C. D. Poole, and R. E. Wagner, "Investigation of polarization dispersion in long lengths of single-mode fiber using multilongitudinal mode lasers," IEEE J. Lightwave Technol., LT-5, 1987, pp. 618-1622, and D. Andresciani, F. Curti, F. Matera, and B. Daino, "Measurement of the group-delay difference between the principal states of polarization on a low-birefringence terrestrial fiber cable," Optics Lett. 12, 1987, pp. 844-846, are reproduced in FIGS. 2 and 3, respectively. The output state of polarization is measured and displayed on a Poincare sphere. As the optical source is tuned over a range of frequencies, the output state of polarization traces out an arc on the sphere. Assuming that the principal states and .tau..sub.PMD are fairly constant over the frequency range, the principal states are located at the center of the arc and diametrically opposite, and .tau..sub.PMD =.alpha./.DELTA..omega., where .alpha. is the arc between two output states of polarization separated by .DELTA..omega., and .alpha. is measured about the axis joining the two principal states of polarization.
This technique also suffers several disadvantages. Again, the principal states of polarization must be found for this technique to work. Finding the principal states is time-consuming and very difficult to automate. If .tau..sub.PMD is small over a particular frequency interval, the arc traced out will be too small to indicate its center, making this technique unusable.
A third technique for measuring polarization mode dispersion, described in C. D. Poole, "Measurement of polarization-mode dispersion in single-mode fibers with random mode coupling," Optics Lett., 14, 1989, pp. 523-525, involves an apparatus such as that reproduced in FIG. 4. In use, the photocurrent is measured as a function of the optical frequency selected by the monochrometer. Quoting from this reference, Poole shows that "if the [polarization] dispersion is both stationary and ergodic, the density of the extrema in the transmission spectrum is directly related to the ensemble average delay " &lt;.tau..sub.PMD &gt;. Under these assumptions, the ensemble average delay time is given by ##EQU1## where N is the number of extrema observed in the interval .DELTA..omega. in the photocurrent versus optical frequency plot.
This technique has the disadvantage that it is applicable only when the polarization mode dispersion is stationary and ergodic. These conditions are often satisfied when measuring polarization mode dispersion in a long single-mode optical fiber, but they are not generally satisfied for all devices or networks, for example, a pigtailed isolator or integrated-optic device. Nevertheless, even when the conditions are satisfied, this technique does not indicate the principal states of polarization, and yields poor resolution in the measured value of &lt;.tau..sub.PMD &gt;.
Finally, another known polarization mode dispersion measurement technique disclosed in C. D. Poole, N. S. Bergano, R. E. Wagner, and H. J. Schulte, "Polarization dispersion and principal states in a 147-km under-sea lightwave cable," IEEE J. Lightwave Technol., LT-6, 1988, pp. 1185-1190, uses the apparatus shown in FIG. 5. Note that this apparatus is identical in function to that of the technique described in connection with FIGS. 2 and 3 above, except that a polarization controller is inserted between the tunable source and the device under test, in this case a 147-km cable. In this context, a polarization controller is an arrangement of loops of single-mode optical fiber, which can be manually adjusted to change its polarization transformation, allowing the user to generate at the output of the loops any desired state of polarization within the constraint that the degree of polarization cannot be changed by the loops; See H. C. LeFevre, "Single-mode fibre fractional wave devices and polarization controllers," Elect. Lett., 16, 1980, pp. 778-780.
In use, the optical source is tuned to approximately measure the derivative ds.sub.1 /d.omega., where s.sub.1 is the normalized Stokes vector representing the state of polarization at the output of the device under test. The polarization controller is then adjusted to generate a new output state of polarization s.sub.2, and the optical source is retuned over exactly the same range to approximately measure ds.sub.2 /d.omega.. The desired characteristics of polarization mode dispersion can be derived from the vector q given by ##EQU2## The normalized Stokes vectors p representing the output principal states are given by p=.+-.q.vertline.q.vertline., and .tau..sub.PMD =.vertline.q.vertline..
However, this technique suffers the disadvantage that large errors in the calculation of q will occur when s.sub.1 or s.sub.2 is near one of the output principal states, and also when the cross product in Equation (2) is small.
Therefore, a method and apparatus for facilitating determination of polarization mode dispersion in an optical device under test to various polarization states of an incident light beam are needed so that the response characteristic of the optical device to different polarization states can be assessed, for example, during the design of the optical device. Moreover, such a polarization mode dispersion determination desirably would be calibrated, accurate, and rapidly obtained, as well as convenient to perform.